An expiration date is just when the company will stop being responsible for the product--meaning, that's the date that they can GUARANTEE the product will be fresh with certainty. A few days is not a big deal at all. Usually most products actually have a couple of months left where they'll still be fine.
I got a giggle out of an expiration date on white vinegar. What is it going to do, turn BACK into alcohol? Its a natural preservative. About the only thing I could find while searching the web was that over time vinegar will lose some of it potency, maybe that is why they put an expiration date on it, or perhaps it is just a marketing ploy to make you buy more vinegar!
Regular apple cider vinegar will ferment and start growing yucky stuff in the bottom of the bottle. But I remember when I was little, seeing my grandma transfer the vinegar to another bottle/cup. She'd trash the fermented gunk and then pour the vinegar right back into the original bottle.
Well, Vinegar is basically a form of alcohol that's spoiled, so the expiration date is just there because I believe a law was passed in the US stating that all food products needed to have one. It won't go bad.
And Pixie - the gunk that your grandmother transferred out is probably vinegar mother. Vinegar is basically the end result when a certain type of bacteria gets into an alcohol and eats the sugars therein. The gunk is a collection of the bacteria which has formed after being there for a while and eating all the sugars to eat. I know that in white and red wine vinegars, if you take that gunk and add it to a cup or two of wine in a loosely covered jar, it will start to make more vinegar. I actually "grow" my own vinegars. If you're a wine person, but never seem to completely finish a bottle, it's a great way to use the left overs.